1. Field of the Invention
The present invention relates to a non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte solution in which a solute is dissolved in a non-aqueous solvent. More particularly, the invention relates to improvements in the non-aqueous electrolyte solution of the non-aqueous electrolyte secondary battery that employs a negative electrode active material capable of alloying with lithium for the negative electrode to increase the charge-discharge capacity, so that the charge-discharge cycle performance and the storage performance in a charged state can be improved.
2. Description of Related Art
In recent years, non-aqueous electrolyte secondary batteries have been in use as power sources for mobile electronic devices and electric power storage. A non-aqueous electrolyte secondary battery typically uses a non-aqueous electrolyte and performs charge-discharge operations by transferring lithium ions between the positive electrode and the negative electrode.
In this type of non-aqueous electrolyte secondary battery, a graphite material has been widely used as the negative electrode active material in the negative electrode.
In the case of the graphite material, the discharge potential is flat and the insertion and deinsertion of lithium ions take place between the graphite crystal layers during charge and discharge. As a result, the graphite material has advantages in that it can inhibit the formation of dentritic metallic lithium and also it shows a small volumetric change associated with charge and discharge.
Meanwhile, significant size and weight reductions in mobile electronic devices such as mobile telephones, notebook computers, and PDAs have been achieved in recent years. Moreover, power consumption of such devices has been increasing as the number of functions of the devices has increased. As a consequence, demand has been increasing for lighter weight and higher capacity non-aqueous electrolyte secondary batteries used as power sources for such devices.
However, when a graphite material is used for the negative electrode active material, the above-mentioned demand cannot be met sufficiently because the capacity is not quite sufficient in the graphite material.
For this reason, the materials that can form an alloy with lithium, such as Si, Zn, Pb, Sn, Ge, and Al, have been used in recent years as high capacity negative electrode active materials.
These materials that can form an alloy with lithium undergo extensive volumetric changes in association with the intercalation and deintercalation of lithium. This can result in electrolyte dry-out in the electrode assembly containing the positive electrode and the negative electrode with separators interposed therebetween, especially in the case of the electrode assembly in which the positive electrode and the negative electrode are wound together with a separator interposed therebetween, since the non-aqueous electrolyte solution retained therein is forced out because of the volumetric changes during charge and discharge. As a consequence, the internal resistance of the battery increases considerably, deteriorating the battery performance such as the charge-discharge cycle performance.
In recent years, in order to inhibit the negative electrode active material made of a material capable of alloying with lithium from the deterioration due to the expansion resulting from charge and discharge, it has been proposed to use a non-aqueous electrolyte solution containing a non-aqueous solvent comprising a fluorinated carbonic ester, as disclosed in Japanese Published Unexamined Patent Application No. 2006-86058.
However, the use of the non-aqueous solvent comprising a fluorinated carbonic ester for the non-aqueous electrolyte solution as described above has the following problems. The viscosity of the non-aqueous electrolyte solution increases, impairing the distribution of the non-aqueous electrolyte solution within the battery. Consequently, the internal resistance increases, and battery performance such as the charge-discharge cycle performance deteriorates.
Also in recent years, it has been proposed, as disclosed in Japanese Published Unexamined Patent Application No. 2004-319212, to use a non-aqueous solvent containing methyl acetate and another chain carboxylic ester, in order to lower the viscosity of the non-aqueous electrolyte solution.
However, chain carboxylic esters such as methyl acetate generally have low electrochemical stability in comparison with chain carbonates such as dimethyl carbonate. For this reason, for example, when the non-aqueous electrolyte secondary battery is left in a charged state under a high temperature environment, the non-aqueous electrolyte solution decomposes, and the discharge characteristics deteriorate considerably.